The first level of Spartan Ops isn’t a whole lot of fun if you’re playing solo, especially on Heroic. The three Warthogs you get at the start get blown up pretty easy, and neither the 'Hogs nor your AI teammates respawn. You end up being reduced to the option of either the Ghost or the Wrait pretty quick.
Why not a few Mantis’ on the map? Even the campaign version isn’t too overpowered for Spartan Ops. And one thing I really hated about Firefight was the scarcity of UNSC vehicles; all we ever got was the Warthog and Mongoose; never the Hornet, the Falcon, or the Scorpion… and they’re some of the most fun vehicles to drive.